from math import cos,sqrt,exp,pi

def fun(x):
    return exp(x)#/sqrt(1-x**2)

n=float(input())

def xk(k,n):
    return cos((2*k+1)/(2*n+2)*pi)

ans=sum(fun(xk(i,n)) for i in range(0,int(n+1)))
ans*=pi/(n+1)
print("%.6f"%ans)